The present invention relates to a lockable rotary attachment device and particularly to a lockable rotary attachment device of the type having an axially threaded attachment member, a knob for rotating the attachment member, and a key controlled clutch mechanism for drivingly interconnecting the knob and the attachment member. Such lockable rotary attachment devices are used for clamping or securing various articles or devices together to impede theft. Some prior lockable rotary attachment devices such as disclosed in U.S. Pat. Nos 1,875,734; 1,970,660; 3,796,074; 4,336,698 and 4,441,344, arrange a key operated lock so that it moves a clutch mechanism into an engagement position to drivingly connect the knob to the threaded attachment member. However, such devices are somewhat tedious to operate since it is frequently necessary to angularly manipulate the rotary knob while turning the key operated lock in order to allow the clutch mechanism to move into an engaged position. Some other lockable rotary attachment devices such as disclosed in U.S. Pat. Nos. 1,786,201; 2,143,925 and 2,534,446 are arranged to lock the knob only in a disengaged position in which the knob can turn freely relative to the rotary attachment member. Further, in some rotary attachment devices, the torque applied to the knob to turn the attachment member is transmitted in whole or in part through the key operated lock and can result in damage to the lock or failure of the lockable attachment device. In addition, prior lockable rotary attachment devices were generally relatively complex and expensive to manufacture.